A variety of agricultural implements typically incorporate a frame wherein the frame height above the ground can be adjusted by an elevation control mechanism. When the frame carries instrumentalities for penetrating the soil, this mechanism functions as a "depth control" mechanism. To this end, it is common to mount one or more wheels of the implement on an axle carried on a lever which is pivotally mounted to the frame. The lever can be set at any desired angle relative to the frame to establish the desired height of the frame above the ground. Typically, the orientation of the lever relative to the frame is established by means of an hydraulic piston-cylinder actuator that is connected at one end to the frame and at the other end to the lever on which the wheel axle is carried. With the above-described type of hydraulic actuator system, it is usually desired to maintain the selected frame elevation for the duration of the particular agricultural work being done--until a greater or lesser elevation is desired because of changing ground conditions or until it is desired to raise the frame to the highest possible elevation for road transport.
One conventional approach to maintaining a selected frame elevation with an hydraulic piston-cylinder actuator system involves the use of a locking collar on the actuator piston rod. The collar is adjusted to limit the stroke depth at the desired frame elevation. However, this requires that the operator leave the operating seat on the machine and physically adjust the locking collar or collars. Further, there are usually at least two piston-cylinder actuators, and the collar on each piston rod must be carefully positioned so that, at the selected frame elevation, the frame is level and not subjected to torsion.
In another conventional system for setting the frame elevation, a more expensive mechanism is provided in the form of a stop valve to control the actuator piston stroke. However, with this type of system, internal leakage can occur. The actuator piston then begins to move from its initially set position as the fluid leaks from the high pressure side of the hydraulic system to the low pressure side. This results in a change of the elevation of the frame.
It would be desirable to provide an improved mechanism for setting the elevation of an implement frame which could be easily adjusted and could maintain the selected elevation of the implement frame for a sustained period of time. It would also be beneficial if such an improved mechanism could be employed with a relatively wide implement frame having a plurality of hydraulic piston-cylinder actuators and could be employed in such a manner so as to more easily maintain the implement frame level at any selected elevation.